Within the last fifty years the art of metal casting has made significant advances with the advent of die casting machines and continuous strip casters. While advances have been made in the die casting machines only a few changes have occurred in the techniques for delivery of the molten metal to the die casting machine. The industry, for the most part, still delivers the molten metal manually to the die casting machine in heated ladles or heated tiltable crucibles.
Hand ladles are commonly used for transporting the molten metal from the melting pot to the mold or the die casting machine. However, hand ladles rely heavily on the experience of the operator to measure exact volumes of metal into a ladle and of transporting the metal to the mold. Hand ladling is limited by the stamina of the operator to pour a large number of parts. It also becomes less practical as the shot size increases. Although automatic ladling with the use of a robot arm is a possibility, it is not used commercially in the magnesium die casting industry. In both systems, the lack of a good mold wash for magnesium and the difficulty of protecting the metal from the atmosphere during transfer from the melting pot to the mold are prime concerns.
Accordingly, a common problem has been the need for more casting capacity and in particular capacity for large parts, such as those which are produced on 1200 ton and larger casting machines. Reliable, economical metal transfer systems for the transport of molten metal in such large quantities has only partially been met by the industry.
One transfer system that is capable of transporting large quantities of molten metal employs the use of gravity to move the molten metal via a siphon tube from the melting pot to a die casting machine. Such a gravity metering system is described in a paper by O. Hustoft and E. Estergaard, entitled "Gravity Metering for Magnesium Cold Chamber Die Casting", delivered at a symposium of the "Society of Die Casting Engineers, Inc." on May 11-14, 1987 in Toronto, Canada.
Although gravity metering systems are in use today and have achieved a measure of success, they still suffer from a number of drawbacks. One obvious drawback lies in the fact that the melting pot must be positioned at an elevated level which poses some danger to the operator due to the higher level of molten metal and movement of the siphoning tube vertically up and down over a shot delivery apparatus for injecting a quantity of molten metal into the die of a die casting apparatus. In addition, the metal level in the furnace must be maintained within tight limits of about plus or minus 0.4 in. (about 1 cm) to achieve consistent metering. Although the gravity metering system is capable of metering molten magnesium at rates of from 1.7 to 2.6 lb/sec. (0.8 to 1.2 Kg/sec.) for making large parts, it is not capable of metering portions of molten magnesium on the order of less than 1 lb for making relatively small parts. The system also requires frequent replacement of a valve for holding or retaining the molten metal in the siphoning tube. The valve usually consists of a simple ball valve which, over a period of time, accumulates a layer of metal on the valve seating surface, thus developing a leak allowing a portion of the molten metal to flow back into the melting pot or allowing the metal to continue flowing into the shot delivery apparatus even when it is not desired.
Other methods for conveying molten metal include the use of systems for pumping the molten metal out of a melting pot and delivering the metal through heated conduits to the die of a die casting apparatus. Critical to the operation of such systems are the pumps that are employed for conveying the molten metal. Various types of pumps can be used, including gas displacement, positive displacement (piston or plunger), centrifugal, or electromagnetic pumps.
Gas displacement pumps use gas pressure to force molten metal from a sealed vessel through a transfer tube which is submerged in the molten metal. The application of a head of pressure to the molten metal in the sealed vessel by the pump forces the metal out of the vessel into the transfer tube and into the shot delivery apparatus. A valve is provided in the transfer tube and may be a simple ball check valve or a pneumatically or hydraulically actuated valve. The valve, when closed, allows the holding pot to be pressurized from an external source of a pressurizing gas. When the valve is opened, pressure exerted onto the molten metal in the pot forces the metal through the transfer conduit into a shot delivery apparatus. A typical valve is disclosed in U.S. Pat. No. 3,726,305 issued to S. C. Erickson et al. The most common problem with this type of pump is in its inconsistent delivery of shots, i.e. varying amounts or volumes of the molten metal to the shot delivery apparatus due to leakage of the molten metal past the check valve seat. Accordingly, a periodic cleaning of the check valve is required, which is difficult and time consuming. Other disadvantages include the fact that small castings of less than 1 lb. can not be made and that the heated transfer tube frequently becomes plugged up with the metal.
Positive displacement pumps have a plunger or piston arrangement which make it difficult to prevent molten metal from becoming lodged between the cylinder and the piston to the extent that the piston can not perform its intended transfer motion. In effect, the piston becomes "stuck" in the cylinder and must frequently be removed from the cylinder for cleaning purposes.
In centrifugal pumps, the required head pressure and flow rate is small, therefore efficiency is not a primary concern. The head pressure is produced by rotating an impeller inside of a circular housing which has a tangential discharge. While centrifugal pumps are effective in moving large quantities of molten metal, they cannot meter small volumes of metal without an improved control scheme. They also suffer from the disadvantage that they have moving parts, i.e. the impeller.
An important consideration in the metal handling arts is that any moving parts for conveying molten metal to a casting machine inherently complicates the system and poses difficult problems in maintaining the system troublefree.
Of the various pumping systems available today, the preferred pump employed in the present invention is an annular linear induction or electromagnetic pump (EM pump) which does not employ any movable parts and which is capable of sustained operation over long periods of time even while the pump is submerged in the molten metal.